Research

The EuroCal research on the most luminous galaxies is focused around four observational and two theoretical tasks.

Observations

Radio and gamma-ray monitoring of blazars. Task leader: J.A. Zensus

The launch of the Fermi Gamma-ray Space Telescope in June of 2008 has provided an unprecedented opportunity for the systematic study of active galaxies (AGN) and in particular blazar jets in GeV energies. The GeV window, which had remained unexplored for over a decade before Fermi, is particularly important for such studies because inverse Compton emission from blazar jets peaks at this range. Fermi has prompted very significant efforts in multi-wavelength blazar monitoring since great potential for understanding the physics of AGNs lies on the study of the timely variable broad-band spectral energy distribution of these systems. In collaboration with the Fermi AGN team, the F-GAMMA program is monitoring monthly the broad-band radio spectra of a prominent sample of about 60 selected Fermi-GST AGN (conducted by the MPIfR) using some state of the art telescopes such as the 100-m Effelsberg, the 30-m IRAM Pico Veleta, and the 12-m APEX telescope covering the band from 2.6 to 345 GHz at 12 frequencies. The Owens Valley Radio Observatory (OVRO) 40m telescope is, on the other hand, monitoring a statistically complete sample of over 1,200 blazars, twice a week at 15 GHz.

The two programs are highly complementary: the OVRO 40 m program is ideal for the study of the typical properties of blazar jets, thanks to its strict and reproducible statistical selection criteria, while the F-GAMMA sample is ideal for the study of exceptional behavior and detailed individual-object studies, thanks to its emphasis on particularly active and prominent (well-studied in all wavelengths) sources. As part of this task, we pursue: an exchange of data analysis algorithms; cross-calibration of observations; tests on the OVRO complete sample of any prominent behavior detected in F-GAMMA sources, and, conversely, verification of any behavior detected in the single-frequency OVRO data using the multi-frequency F-GAMMA data.

Optical data are a vital counterpart to radio and gamma-ray data in the study of blazar jets, as the synchrotron energy flux from blazar jets frequently peaks at optical wavelengths.The 1.3 m telescope at the Skinakas Observatory is a uniquely suitable facility for studying these bright and highly variable sources: it is a well-equipped modern telescope, it is built at a site with excellent seeing (comparable to the best observing sites world-wide), it can be remotely operated, and it can provide large amounts of observing time (four nights a week, between May and October when the site is accessible) for blazar monitoring. Variability monitoring both in optical and in the near infrared is possible. For this reason, AGC and Caltech researchers have an ongoing collaboration on blazar optical photometry and variability, with the first observations already having taken place and the analysis currently underway. In addition to the importance of photometric monitoring, polarimetric monitoring in optical wavelengths can also reveal valuable information about blazar jets. Blazars are known to be highly linearly polarized at optical wavelengths, and 50% of them are expected to be more than 2% polarized. Optical polarization observations, especially during outbursts, probe the magnetic field structure in the jet and the evolution of disturbances responsible for blazar flares. The University of Crete/FORTH, the MPIfR and Caltech are pursuing a joint effort that will combine optical photometric and polarimetric monitoring of gamma-ray loud blazars detected by Fermi, using the 1.3 m telescope of the Skinakas Observatory in Crete and, for the case of the polarimetric monitoring, a specially-built optical/near-infrared polarimeter.

Even after the end of the Fermi mission, the polarimetric camera can be used in conjunction with TeV blazar monitoring by planned ground-based gamma-ray telescopes such as the Cherenkov Telescope Array (CTA), but also in studies of the magnetic field in translucent clouds, a subject of high interest for the star-formation and interstellar-medium groups at Crete and Caltech.

A key characteristic that provides a number of possibilities of long-term collaboration is that Skinakas Observatory has access to similar part of the northern sky and it is at 10-12hrs time difference with the telescope facilities Caltech operates in California and Hawaii. This enables projects requiring nearly continuous observations of astronomical targets.

One of the defining properties of AGN is that of copious emission in X-rays. In fact, we now believe that emission of X-rays at a rate of 1042 erg s-1 (or higher) in the 2-10 keV band from the central parts of a galaxy, signifies in itself the presence of an “active nucleus”, i.e. of a SMBH which swallows large quantities of nearby gas (or even stars). Friction and other forces can heat the inner part of the disk to millions of degrees, hence the large X-ray flux emitted by these objects. Study of the X-ray emission from these objects is one of the few direct ways we have to study matter's behaviour right outside the black hole's horizon, i.e. in the exotic environment of strong gravity (one of the most active research areas in modern Astronomy today).

Although the “soft” X-ray sky (at energies lower than 10 keV) has been extensively studied the last 30 years (with the use of state-of-the art observatories such as ESA's XMM-Newton and NASA's Chandra), the study of the “hard” X-ray Universe, at higher energies, has been less successful. The main reason was the inability of all previous orbiting telescopes to focus the X-ray photons on the detector's plane. And yet, in order to study the intrinsic X-ray emission from AGN, one must study them at these high energies, as absorption effects at energies below ~ 6-10 keV (from intervening neutral and/or ionized material) make it difficult to perform such studies. “NuSTAR” (the “Nuclear Spectroscopic Telescope Array” ) is a NASA mission scheduled for launch in February 2012. The NuSTAR mission will deploy the first focusing telescopes to image the sky in the high energy X-ray (5 - 80 keV) region, providing more than two orders of magnitude improvement in sensitivity compared to previous high energy missions. Consequently, NuSTAR will allow, better than ever before, the detailed study of the X-ray energy spectrum, but also of the variability properties, of AGN.

The Infrared Astronomical Satellite (IRAS) provided the first unbiased survey of the sky at mid and far-infrared wavelengths, giving us a comprehensive census of the infrared emission properties of galaxies in the local Universe. A major result of this survey was the discovery of a large population of luminous and ultraluminous infrared galaxies (U/LIRGs) which emit a significant fraction of their bolometric luminosity in the far-infrared. Even though rare in the local Universe, ULIRGs still outnumber the bright quasars. LIRGs cover the full range of morphologies, from single isolated disk galaxies to interacting systems and advanced mergers, exhibiting enhanced star-formation rates and a higher fraction of Active Galactic Nuclei (AGN) compared to less luminous galaxies. A detailed study of low-redshift LIRGs is critical for our understanding of the cosmic evolution of galaxies and SMBHs, since (1) LIRGs comprise the bulk of the cosmic infrared background and dominate the star-formation activity between 0.5 < z < 1, and (2) AGN fueling and mass accretion onto a central SMBH may preferentially occur during episodes of enhanced nuclear star-formation helping to naturally explain the scaling of SMBHs and stellar bulge masses. However, probing the nuclear properties of these systems using well-established optical diagnostic methods is challenging, since they harbor large amounts of molecular gas and dust making them optically thick in the visible part of the spectrum. Often even X-rays cannot penetrate the most obscured nuclei, which appear as Compton thick.

The advent of the Infrared Space Observatory and more recently of the Spitzer Space Telescope, whose superb sensitivity enabled mid-infrared spectroscopic observations of large samples of galaxies, opened a new window in our understanding of near-by and distant LIRGs. With the Great Observatories All-sky LIRG Survey, we are currently measuring the properties of a large, complete sample of low-redshift LIRGs across the electromagnetic spectrum using Chandra, GALEX, HST, Spitzer, CARMA, and the VLA. The GOALS targets are drawn from the IRAS Revised Bright Galaxy Sample, a complete sample of 629 galaxies with IRAS S60 > 5.24 Jy and Galactic latitudes |b| > 5 degrees. The 629 galaxies have a median redshift of z = 0.008. There are 179 LIRGs and 23 ULIRGs in the RBGS, and these galaxies define the GOALS sample. Using Spitzer IRS spectra, we have recently quantified for the first time the contribution of AGN to the power output of LIRGs across the entire GOALS sample, and we estimate that while AGN are present in nearly 25% of the sample, these AGN contribute only about 10 − 15% of the total bolometric energy from LIRGs as a class. The vast majority of the energy produced by LIRGs and reradiated in the far-infrared is produced as a result of star formation. The fact that most of this energy is produced at the very central regions of these galaxies has direct consequences in the mid-IR spectral characteristics, as well as in their optical and near-IR morphology as probed by Hubble Space Telescope.

In spite of intensive observational efforts over the last four decades, the details of the structure and composition of blazar jets, and of the mechanisms through which jets are launched, accelerate non-thermal particle populations, and emit radiation, remain elusive. The reason is that small variations in the angle between the jet axis and the line of sight result in a large range of observed properties, such as apparent luminosity, variability, and energy spectrum. As a result the study of large, carefully selected, samples is necessary to determine the physical processes and conditions of the parent population. In order to overcome the difficulties associated with constraining blazar physics, there is the need for a phenomenological bridge between simulations and observations, which will allow us to best use the multitude of observational data now available to us in a manner that will most effectively test theoretical models. Current data offer an unprecedented coverage in the frequency and time domains, as well as additional, independently measured blazar properties (such as superluminal motions, black hole masses, host redshifts, and jet magnetic field properties accessible through polarization observations). For this reason the timing is especially opportune for progress in the field of jet astrophysics, using as a laboratory blazars, which are sources for which relativistic effects have their most extreme expression.

The F-GAMMA, OVRO 40 m, and Fermi data are a perfect test-bed for such phenomenological models of the blazar population: their combination offers a statistically complete survey, multifrequency coverage, and information in the time domain. For this reason, they can be used to test theoretical predictions connecting blazar observables, such as the observed flaring ratios, variability amplitudes, and duty cycles of blazars (including the frequency dependence of these quantities) with properties of the central engine (black-hole mass and accretion rate, jet Doppler factor and orientation). The recently funded optical polarimetry program at the Skinakas Observatory will further provide independent constraints for the magnetic field and the structure of blazar jets. In addition to population studies, the multi-frequency time-domain data can be used for time-dependent spectral modeling of blazars, which can constrain blazar emission and jet structure models on an object-to-object basis.

Theoretical modeling of star-forming regions. Task leader: K. Tassis

Star formation theory is a subject that has been intensively pursued over many decades. The physics involved is diverse and complex, and it includes gravity, turbulence, magnetic fields, radiative transfer and feedback processes, interstellar chemistry, cosmic-ray physics, and processes involving interstellar dust grains. However, the relative importance of each physical process that drives and opposes the formation of stars remains an open question and a subject of heated debate in the field. The reason is that physical conditions in star-forming regions are difficult to extract from observable quantities, such as dust continuum emission or molecular line maps: each observable is typically affected by multiple factors, which are nontrivial to be decoupled.

A most promising way to make progress is to create models that simultaneously and self-consistently follow all processes affecting interstellar gas probes (such as molecular spectral lines and dust continuum emission and absorption); to vary all uncertain or unknown parameters within limits allowed by observations; to produce theoretical predictions of the observables (such as molecular line profiles, molecular line and dust continuum maps, and line strength ratios); and to identify these diagnostics that feature minimal degeneracies between different theoretical dynamical models and, as a result, maximal potential for discrimination between theories of star formation.

In the case of exceptionally luminous star-forming galaxies, the existence of a well-studied, over many wavelengths, complete sample of galaxies, such as GOALS discussed above, emphasizes the need for detailed star-forming region models that can benefit from the large number of available observables to push the field of star-formation theory forward.

Project EUROCAL is supported by the European Union’s Seventh Framework Programme, through an International Research Staff Exchange Scheme (IRSES) Marie Curie Action, under grant agreemen PIRSES-GA-2012-316788.